1. Field of the Invention
The present invention relates to the use of polysuccinimide as an additive to cementitious materials that improves flowability during mixing and leads to improved strength on curing. In addition, polyaspartate alone, polyaspartate in combination with polysuccinimide, or a copolymer of aspartate and succinimide residues also are useful in cement for improved flowability over time and development of strength.
2. Discussion of the Background
Polysuccinimide is made commercially by thermal polymerization of maleic anhydride and ammonia and related precursor monomeric reactants that upon heating produce aspartic acid which then condenses to yield the polyimide. (U.S. Pat. No. 3,846,380 to Fujimoto and Teranishi, 1974; U.S. Pat. No. 4,839,461 to Boehmke, 1989; U.S. Pat. No. 5,219,952 to Koskan and Meah, 1993; and U.S. Pat. No. 5,371,180 to Groth et al., 1994).
In addition, thermal polymerization of aspartic acid itself produces polysuccinimide. This is accomplished by use of the dry powder of aspartic acid alone (Schiff, 1898; U.S. Pat. No. 5,051,401 to Sikes, 1991; U.S. Pat. No. 5,152,902 to Koskan et al., 1992) or in the presence of an acidic catalyst/solvent such as phosphoric acid (U.S. Pat. No. 3,052,655 to Fox and Harada, 1962; U.S. Pat. No. 4,363,797 to Jacquet et al., 1982; U.S. Pat. No. 5,329,020 to Kalota and Martin, 1994; U.S. Pat. No. 5,393,868 to Freeman et al., 1995; and U.S. Pat. No. 5,536,813 to Charpenel and Lepage, 1996). ##STR1##
Typically, the synthesis of polyaspartic acid does involve polysuccinimide as an intermediate, which is then subjected to mild alkaline hydrolysis to produce polyaspartate, usually as the sodium salt. However, it also is possible to produce polyaspartate directly, without the intermediate polysuccinimide, by thermal polymerization of alkali salts of aspartate, such as monosodium aspartate (U.S. patent application Ser. No. 08/842,016 by Sikes and Japanese Kokai 08-169741. This publication is not prior art with respect to the invention disclosed and claimed herein). Similarly, sodium maleamic acid has been used to produce polyaspartate directly (U.S. Pat. No. 5,548,036 to Kroner et al., 1996). IL addition, mixtures of maleic acid, urea, and sodium carbonate were treated thermally to produce polyaspartate directly (U.S. Pat. No. 5,371,180 to Groth et al., 1996).
It is further possible to produce a mixed amide/imide copolymer of aspartic acid and succinimide either by use of temperatures that are too low to drive the reaction to completion (e.g., &lt;140.degree. C., U.S. Pat. No. 4,839,461 to Boehmke, 1989; U.S. Pat. No. 5,219,952 to Koskan and Meah, 1993) or by use of reaction times that are too short (e.g., &lt;10 minutes at 180.degree. C., U.S. Pat. No. 5,493,004 to Groth et al., 1996). Finally, it is possible to produce a mixed amide/imide copolymer of aspartate and succinimide by thermal polymerization even at elevated temperature for extended reaction times by use of a reaction mixture of an alkali salt of aspartate such as monosodium aspartate and aspartic acid (U.S. patent application Ser. No. 08/842,016 by Sikes).
The hydrolysis of polysuccinimide to open the imide rings and produce polyaspartate is rate-dependent on the particle size of the polysuccinimide and proceeds over periods of minutes to hours at pH around 10 and temperatures around 80.degree. C. (Mosig, 1992; Mosig et al., 1997; Low et al., 1996). The hydrolysis may take more than 24 hours at pH 10, 25.degree. C. (U.S. patent application Ser. No. 08/596,439 by Sikes). Early kinetic analysis of the hydrolysis was interpreted to show that the reaction slowed as the ring-opening proceeded due to repulsion of polyanionic regions of polyaspartate with hydroxide ion (Hoagland and Fox, 1973), perhaps resulting in a mixed polymer of succinimide and aspartate residues. However, NMR (nuclear magnetic resonance) spectra of reaction products later suggested that the ring-opening of individual polymer molecules proceeded rapidly to completion once begun such that there was only a set of completely ring-opened polyaspartate molecules along with a residual set of core, insoluble polysuccinimide particles that had yet to react with the alkaline, aqueous phase (Wolk et al., 1994). Whereas the polysuccinimide particles are hydrophobic and highly insoluble in water, the polyaspartate molecules are highly hydrophilic and will form aqueous solutions of up to 40% by weight.
The conversion of polysuccinimide to polyaspartate in mild, alkaline, aqueous solution in effect is a controlled release of soluble, polyanionic polymer from an insoluble hydrophobic particle. This is a useful reaction to occur within the environment of forming cementitious materials, which becomes alkaline as the cementitious mineral phases react with the mixing water to form the hydration products of set cement, which include Ca(OH).sub.2. The presence of both hydrophobic and polyanionic polymers in controlled amounts are known in the art to improve the properties of cementitious materials both during the forming stages and in the finished products.
For example, polymers that have both hydrophobic and hydrophilic character are useful in cement for dispersing the mineral particles during mixing, improving the flowability of the mix, and allowing reduction in the amount of water required to form the mix. At the same time, properties such as setting time, adhesion, and strength of the formed cementitious materials remained favorable or were improved (U.S. Pat. No. 5,290,869 to Kinoshita et al., 1994; U.S. Pat. No. 5,362,324 to Cerulli et al., 1994; U.S. Pat. No 5,432,212 to Honda et al., 1995; U.S. Pat. No. 5,466,289 to Yonezawa et al., 1995; and U.S. Pat. No. 5.494,516 and U.S. Pat. No. 5,609,681 to Josef et al., 1994, 1996). Monomeric constituents of the polymers of these references include, for example, alkylene oxide, napthalene sulfonate-formaldehyde, styrene, ethylene glycol, propylene glycol, maleic anhydride, and methacrylate. Polysuccinimide itself has been reported to improve the flowability of Portland cement powder when mixed with water (Japanese Kokai 008-169741 to Harada et al.; this publication is not prior art with respect to the invention disclosed and claimed herein).
Thus, it would be useful if a single homopolymer could provide the improved flowability of a cement mixture provided by an hydrophobized polymer additive as well as the improved binding characteristics provided by polyanionic polymers within the cement mixture. It is here reported that polysuccinimide may be added to cementitious materials for the purposes of improving flowability during mixing, decreasing slump loss during transport and placement, and strengthening the formed product. The phrase, "cementitious materials", is meant to include: cement, a mixture of cement powder and water; mortar, a mixture of cement powder, fine aggregates, and water; and concrete, a mixture of cement powder, fine aggregates, and coarse aggregates; as well as any other combination of cement powder and other materials such as silica fame, fly ash, slag, and lime powder.
In the aqueous environment of freshly mixed cementitious materials, polysuccinimide is converted to polyaspartate. This provides both hydrophobic and hydrophilic characteristics to the interactions of the polymer with the hydrating cement phases during the period of mixing and setting.